It has been known in the past to arrange a catalyst and air-fuel ratio sensor in an exhaust passage of an internal combustion engine and suppress deterioration of the exhaust emission by controlling the air-fuel ratio of the inflowing exhaust gas flowing into the catalyst based on the output of the air-fuel ratio sensor.
For example, in the internal combustion engine described in PTL 1, if the air-fuel ratio detected by a downstream side air-fuel ratio sensor arranged at a downstream side of a catalyst reaches a rich judged air-fuel ratio richer than a stoichiometric air-fuel ratio, a target air-fuel ratio of the inflowing exhaust gas is set to a lean set air-fuel ratio leaner than the stoichiometric air-fuel ratio. On the other hand, if an estimated value of an oxygen storage amount of the catalyst reaches a judged reference storage amount, the target air-fuel ratio of the inflowing exhaust gas is set to a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio. The estimated value of the oxygen storage amount of the catalyst is calculated based on the output of an upstream side air-fuel ratio sensor arranged at an upstream side of the catalyst.
Further, in the internal combustion engine described in PTL 1, a voltage applied to the downstream side air-fuel ratio sensor is controlled so as to precisely detect the air-fuel ratio of the outflowing exhaust gas flowing out from the catalyst. Specifically, when the target air-fuel ratio of the inflowing exhaust gas is richer than a reference air-fuel ratio, the voltage applied to the downstream side air-fuel ratio sensor is made a voltage higher than the voltage at which the output current becomes zero when the air-fuel ratio of the outflowing exhaust gas is the stoichiometric air-fuel ratio. On the other hand, when the target air-fuel ratio of the inflowing exhaust gas is leaner than the reference air-fuel ratio, it is made a voltage lower than the voltage at which the output current becomes zero when the air-fuel ratio of the outflowing exhaust gas is the stoichiometric air-fuel ratio.